The invention relates to a metal halide lamp comprising a discharge vessel having a ceramic wall, which discharge vessel encloses a discharge space comprising an ionizable filling which, in addition to Hg, comprises an alkali iodide of Na or Li or a combination thereof.
A lamp of the type described in the opening paragraph is known from WO 99/28946-A1. The known lamp combines a high specific luminous flux with fairly good color properties (for example, a value of the general color-rendering index Raxe2x89xa760 and a color temperature Tc in the range of 3000 K and 6000 K).
In this lamp, use is made of the recognition that a good color rendition is possible when Na halide is used as a filling constituent of a lamp and when there is a strong broadening and reversal of the Na emission in the Na-D lines during lamp operation. This requires a high temperature of, for example 1170 K (900xc2x0 C.) of the coldest spot Tkp in the discharge vessel. When the Na-D lines are reversed and broadened, they take the form of an emission band in the spectrum, with two maxima at a mutual distance xcex94xcex.
The requirement for a high value of Tkp has the result that the discharge vessel is relatively small, which leads to a high temperature of the wall of the discharge vessel in the practical lamp. The required high temperature excludes the use of quartz or quartz glass for the wall of the discharge vessel and necessitates the use of ceramic material for the wall of this vessel.
In this description and the claims, a ceramic material wall is understood to mean both a wall of metal oxide such as, for example, sapphire or densely sintered polycrystalline Al2O3, and metal nitride, for example AlN.
The light emitted by the lamp has a color point with co-ordinates (x, y) which deviates from that of a blackbody. The mathematical collection of color points of blackbodies is referred to as the blackbody line (BBL). An application of the known lamp as a light source with a Tc above 4700 K has the drawback that the color point of the lamp with co-ordinates x, y is more than 0.05 scale division above the BBL. Consequently, the known lamp is less suitable for use as, for example, a studio lamp.
It is an object of the invention to realize a lamp of the type described in the opening paragraph, in which the drawback described is eliminated.
According to the invention, a lamp of the type described in the opening paragraph is therefore characterized in that the filling of the discharge vessel also comprises TbI3 or GdI3 or a combination thereof.
A lamp according to the invention has the advantage that a value of the color temperature Tc above 4700 K, preferably above 5000 K can be realized and that a general color-rendering index Ra of at least 85 can be realized, while the lamp has a color point which is less than 0.02 above the BBL. The filling preferably comprises TbI3 in a molar ratio of at least 5% and at most 45%. This contributes to a satisfactory stability of the color properties of the lamp during its lifetime. A similar advantage can be realized if the filling comprises GdI3 in a molar ratio of at least 5% and at most 45%.
In a further embodiment, the filling comprises NaI and TlI. A lamp can then be obtained which emits light at a color temperature Tc of between 5500 K and 7600 K and a general color-rendering index Ra of between 85 and 96, with a relatively long lifetime and a relatively small decline of the luminous flux during the lifetime.
In a further embodiment of the lamp according to the invention, the filling comprises CeI3 and TbI3. A lamp can then be realized with a color temperature Tc of between 4700 K and 7500 K and a relatively large luminous flux. To this end, the filling of the lamp preferably comprises NaI, CeI3, ErI3 and TbI3. When the filling comprises TbI3 in a molar ratio of at least 8% and at most 16%, this will contribute to the stability of the color temperature Tc of the lamp during its lifetime. This also generally leads to a smaller shift of the color point of the lamp.
In a further variant, the filling of the lamp comprises LiI, CeI3 and TbI3, which results in a satisfactory stability of the color properties of the lamp in the case of different lamp positions.
The lamp according to the invention appears to be very suitable for use as a light source for, inter alia, video recordings for which a color temperature of more 4700 K, preferably above 5000 K is desired.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.